A known magnetic recording medium for use with a fixed magnetic disc drive is constructed as shown in FIG. 2 by way of example. To produce such a recording medium, a non-magnetic metal layer 12 is formed on a non-magnetic substrate 11, to provide a non-magnetic base 1, and a non-magnetic metal base layer 2 is laminated on the base 1. On the non-magnetic metal base layer 2 is then formed a thin film of magnetic layer 3 made of a ferromagnetic alloy, such as Co--Cr--Ta (cobalt--chromium--tantalum), or Co--Cr--Pt (cobalt--chromium--platinum). Further, a carbon protective layer 4 is formed on the magnetic layer 3, and a lubrication layer 5 made of a liquid lubricant is applied by coating onto the carbon protective layer 4, to thus form a magnetic disc.
The non-magnetic base 1 may be an alumite base, a glass base, or a ceramic base, or may be obtained by forming the non-magnetic metal layer 12 made of Ni--P on the non-magnetic substrate 11 made of an Al--Mg alloy by electroless plating. The base 1 may be ground, or formed with minute protrusions and recesses by texturing. The non-magnetic base 1 thus obtained is fed through a furnace under an Ar atmosphere, while being heated to about 200.degree. C., so that the non-magnetic metal base layer 2 made of Cr, magnetic layer 3, and carbon protective layer 4 made of amorphous carbon are successively formed on the base 1 by sputtering. The carbon protective layer 4 is then coated with a lubricant containing perfluoropolyether (perfluoro polyester).
When the above-described magnetic recording disc is installed in the fixed magnetic disc drive that employs a contact-start-stop (CSS) system to read and write information, the magnetic disc is repeatedly brought into contact with a recording head of the disc drive during the CSS operation. In this CSS system, the recording head is in contact with a surface of the magnetic disc while the disc drive is not operated, and, only when the drive is operated, the recording head floats slightly above the surface of the magnetic disc, so as to read or write information.
The carbon protective layer 4 and the lubrication layer 5 containing perfluoropolyesther are formed on the magnetic layer 3 so as to protect this layer 3 from friction due to such sliding movements of the magnetic disc relative to the recording head.
In the case of a magnetic disc having a relatively small diameter of not larger than 5 inches, the protective layer is generally made of a carbon material, which is formed into a film by sputtering under an Ar atmosphere. The carbon protective layer is favorably used since the amorphous carbon (a-C) layer formed by sputtering has relatively strong graphitic properties, and thus shows a relative low coefficient of friction under an atmosphere containing water, which is a typical property of graphite.
The carbon protective layer as described above has a sufficiently high wear resistance with respect to a conventional Mn--Zn ferrite head (Vickers hardness: about 650), and is highly resistant to the friction between the magnetic disc and the head during a CSS operation. This carbon protective layer, however, has a lower hardness than a ceramic material (Vickers hardness: about 2000), such as Al.sub.2 O.sub.3.Ti or CaTiO.sub.3, which is used for a slider of a thin-film head or MIG head employed in recent fixed magnetic disc drives. Thus, the protective layer tends to wear due to contact with the recording head formed of such a ceramic material, and the head may crash in some cases. To solve these problems, the protective layer used in recent years has developed properties similar to those of diamond having extremely high hardness, to provide a diamond-like carbon (DLC) protective layer in which the percentage of carbon atoms bonded in a diamond structure is higher than that of carbon atoms bonded in a graphite structure.
As a liquid lubricant, there has been used a perfluoropolyether containing lubricant having polar terminal groups, so as to prevent contaminants from being adsorbed onto adsorption sites (functional groups) appearing on the surface of the carbon (DLC) protective layer. If the molecular weight of perfluoropolyether is too low, the chain molecule has a small length, and the coefficient of friction is undesirably increased. If the molecular weight is too high, the chain molecule tends to have an excessively long free end, and the head tends to adhere to the lubrication layer. In view of these, the weight average molecular weight of the perfluoropolyether containing lubricant is favorably within the range of about 2000-5000 MW (molecular weight), and the length of the chain molecule is experimentally controlled to about 30 .ANG.. Since the use of a lubrication layer having a large thickness may induce the recording medium to absorb an excessive lubricant on the medium surface, the thickness of the lubrication layer is desirably controlled to several dozens of angstroms (.ANG.) that is about equal to the length of one molecule. If the carbon protective layer is coated with the lubricant, with a thickness substantially equal to the length of one molecule of the lubricant having a relatively large molecular weight, it is difficult to uniformly cover the entire area of the protective layer with the lubrication layer having such a thickness. As a result, uncovered areas or gaps are left between the macromolecules of the lubricant.
Further, the residue (on the side of the free end) of the molecule of the perfluoropolyether containing lubricant, other than the terminal group bonded to the adsorption site on the surface of the carbon protective layer, has a hydrophobic property. When the carbon protective layer is coated with the lubricant, therefore, the contact angle with water (as measured by a liquid-drop method for measuring the surface tension) is around 90.degree., namely, the coated surface of the recording medium has a large surface tension, and shows poor wettability on the surface of the carbon protective layer. In the presence of the above-described gaps between the molecules of the lubricant, the top surface of the magnetic recording medium, when closely observed, includes mutually isolated molecules of the liquid lubricant having hydrophobic surfaces, which are dispersed on the surface of the carbon protective layer.
In the meantime, recent magnetic disc drives using the magnetic recording medium as described above have a completely enclosed structure wherein the interior space is isolated or sealed from the exterior. Due to this structure, the interior space is easily filled up with gases generated from components inside the disc drive. When the magnetic disc drive is used under a condition of high humidity, in particular, the gases from the interior components are likely to be dissolved into water generated due to the high humidity or dew formation, to thereby produce acids. These gases can be easily identified by analyzing the components inside the magnetic disc drive by gas chromatography. As a result of such an analysis, gases of oxalic acid, formic acid, organic acids, such as acrylic acid, and inorganic acids, such as SO.sub.2 and Cl were detected.
If the magnetic recording medium is used under the atmosphere as described above, the acid gases are first adsorbed or attached to gap portions of the medium surface between the molecules of the lubricant, and are dissolved into adsorbed water under a condition of high humidity, to produce acids. Once the density of the acids becomes higher than a predetermined level, the acids act on ether linkages of the perfluoropolyether forming the lubricant, causing oxidation and reduction, whereby the chain molecules having a high molecular weight are decomposed into low molecules of oxalic acid or formic acid. As a result of the decomposition of the liquid lubricant into low molecules, the adhesiveness between the liquid lubricant and the medium surface is lowered or eliminated, and the lubricant is more likely to be transferred to the magnetic head. Consequently, the magnetic head is contaminated, and its floating characteristics are affected (the floating height is increased), with a result of deterioration of the reproducing capability of the head. Further, the lubricant thus formed into low molecules does not maintain desired lubricating characteristics any longer, and the magnetic head may crash due to its contact with the recording medium in the worst case.